Vulnerable Systems:
* Cisco IOS and Cisco IOS XE with support for RFC4893 and configured for BGP routing.
* Cisco IOS XE Software with support for RFC4893 and configured for BGP routing.

Immune Systems:
* Cisco IOS XR Software
* Cisco IOS NX-OS

Successful exploitation of the vulnerabilities described in this document may result in a reload of the device. The issue could result in repeated exploitation to cause an extended DoS condition.

These vulnerabilities affect only devices running Cisco IOS Software with support for four-octet AS number space (here after referred to as 4-byte AS number) and BGP routing configured.

The first vulnerability could cause an affected device to reload when processing a BGP update that contains autonomous system (AS) path segments made up of more than one thousand autonomous systems. The second vulnerability could cause an affected device to reload when the affected device processes a malformed BGP update that has been crafted to trigger the issue. Cisco has released free software updates to address these vulnerabilities.

The Cisco PSIRT is not aware of malicious exploitation of either of these vulnerabilities, although we are aware of some customers who have seen the first vulnerability triggered within their infrastructures. Further investigation of those incidents seems to indicate that the vulnerability has been accidentally triggered.

These vulnerabilities were discovered via internal product testing.

A Cisco IOS software version that has support for RFC4893 will allow configuration of AS numbers using 4 Bytes. The following example identifies a Cisco device that has 4 byte AS number support:

Router#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)#router bgp ?
<1-65535> Autonomous system number
<1.0-XX.YY> 4 Octets Autonomous system number

Or:

Router#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)#router bgp ?
<1-4294967295> Autonomous system number
<1.0-XX.YY> Autonomous system number

The following example identifies a Cisco device that has 2 byte AS number support:

Router#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)#router bgp ?
<1-65535> Autonomous system number

A router that is running the BGP process will contain a line in the configuration that defines the autonomous system number (AS number), which can be seen by issuing the command line interface (CLI) command "show running-config".

The canonical textual representation of four byte AS Numbers is standardized by the IETF through RFC5396 (Textual Representation of Autonomous System (AS) Numbers). Two major ways for textual representation have been defined as ASDOT and ASPLAIN. Cisco IOS routers support both textual representations of AS numbers. For further information about textual representation of four byte AS numbers in Cisco IOS Software consult the document "Explaining 4-Byte Autonomous System (AS) ASPLAIN and ASDOT Notation for Cisco IOS" at the following link:http://www.cisco.com/en/US/prod/collateral/iosswrel/ps6537/ps6554/ps6599/white_paper_c11_516829.html

Cisco IOS Software with support for RFC4893 is affected by both vulnerabilities if BGP routing is configured using either ASPLAIN or ASDOT notation.

The following example identifies a Cisco device that is configured for BGP using ASPLAIN notation:

router bgp 65536

The following example identifies a Cisco device that is configured for BGP using ASDOT notation:

router bgp 1.0

To determine the Cisco IOS Software release that is running on a Cisco product, administrators can log in to the device and issue the show version command to display the system banner. The system banner confirms that the device is running Cisco IOS Software by displaying text similar to "Cisco Internetwork Operating System Software" or "Cisco IOS Software." The image name displays in parentheses, followed by "Version" and the Cisco IOS Software release name. Other Cisco devices do not have the show version command or may provide
different output.

The following example identifies a Cisco product that is running Cisco IOS Software Release 12.3(26) with an installed image name of C2500-IS-L:

Additional information about Cisco IOS Software release naming conventions is available in "White Paper: Cisco IOS Reference Guide" at the following link: http://www.cisco.com/warp/public/620/1.html

RFC4271 has defined an AS number as a two-octet entity in BGP.
RFC4893 has defined an AS number as a four-octet entity in BGP.

The first vulnerability could cause an affected device to reload when processing a BGP update that contains AS path segments made up of more than one thousand autonomous systems. If an affected 4-byte AS number
BGP speaker receives a BGP update from a 2-byte AS number BGP speaker that contains AS path segments made up of more than one thousand autonomous systems, the device may crash with memory corruption, and
the error "%%Software-forced reload" will be displayed.

The following three conditions are required for successful exploitation of this vulnerability:

* Affected Cisco IOS Software device is a 4-byte AS number BGP speaker
* BGP peering neighbor is a 2-byte AS number BGP speaker
* BGP peering neighbor is capable of sending a BGP update with a series of greater than one thousand AS numbers

The second vulnerability could cause an affected device to reload when the affected device processes a malformed BGP update that has been crafted to trigger the issue. The following three conditions are required for successful exploitation of this vulnerability:

Workaround
For the first vulnerability, there are no workarounds on the affected device. Neighbors could be configured to discard routes that have more than one thousand AS numbers in the AS-path segments. This configuration will help prevent the further propagation of BGP updates with the AS path segments made up of greater than one
thousand AS numbers.

Note: Configuring "bgp maxas-limit [value]" on the affected device does not mitigate this vulnerability.

For the second vulnerability, configuring "bgp maxas-limit [value]" on the affected device does mitigate this vulnerability. Cisco is recommends using a conservative value of 100 to mitigate this vulnerability.